The invention relates to packet-switched data transmission and more precisely to optimization of data packet numbering, particularly in connection with a reliable (acknowledged) transmission.
In the development of the so-called third generation mobile communication systems, for which at least the terms UMTS (Universal Mobile Telecommunication System) and IMT-2000 (International Mobile Telephone System) are used, one starting point has been that they would be so compatible as possible with the second generation mobile communication systems, such as the GSM (Global System for Mobile Communications). The UMTS core network, for instance, is planned to be implemented on the basis of the GSM core network, and thus the already existing networks can be utilized as efficiently as possible. Further, one aim is to enable the third generation mobile stations to utilize a handover between the UMTS and the GSM. This applies to packet-switched data transmission as well, particularly between the UMTS and the GPRS (General Packet Radio Service), which is planned to be used in the GSM.
In packet-switched data transmission, a reliable, i.e. acknowledged, transmission or an unreliable, i.e. unacknowledged, transmission can be used. In the reliable data transmission, the recipient transmits an acknowledgement of the received data packets PDU (Protocol Data Unit) to the transmitter, and the transmitter can transmit the lost or the faulty data packets anew. In the GPRS system, the sub-protocol LLC (Logical Link Control) of the GPRS is responsible for the reliability and acknowledgement of data packet transmission. In an inter-SGSN (Serving GPRS Support Node) handover in the GPRS system, data transmission reliability is ensured by means of a convergence protocol SNDCP (Sub-Network Dependent Convergence Protocol) above the LLC protocol. An 8-bit N-PDU number (Network PDU) is associated with data packets, and on the basis of this number the data packets transmitted to the recipient can be checked.
In the UMTS according to the current specifications, reliability in packet-switched data transmission is ensured by an RLC sequence number of the RLC layer (Radio Link Control) of the packet data protocol. In this respect, the RLC layer of the UMTS corresponds to the LLC layer of the GPRS. In the UMTS, reliability in a handover between the serving nodes is ensured by means of the convergence protocol PDCP (Packet Data Convergence Protocol) above the RLC layer. On the PDCP layer of the UMTS, a 16-bit data packet number is associated with the data packet of the convergence protocol layer PDCP, and this PDCP-PDU number forms a data packet number corresponding logically to the N-PDU number of the GPRS, and on the basis of this number it is checked in the handover that all data packets have been transferred reliably.
In the handover from the GPRS to the UMTS, 8-bit N-PDU numbers are converted in the serving node 3G-SGSN supporting the UMTS to 16-bit PDCP-PDU numbers, which are used for acknowledging the received data packets. Correspondingly, in the handover from the UMTS to the GPRS, 16-bit PDCP-PDU numbers are converted in the serving node 3G-SGSN to 8-bit N-PDU numbers, which are transmitted to the serving node 2G-SGSN of the GPRS and which are used correspondingly for acknowledging the data packets. 8-bit N-PDU numbers are converted to 16-bit PDCP-PDU numbers by extending the value of the N-PDU number by eight most significant bits, each one having the value zero. The conversion of 16-bit PDCP-PDU numbers to 8-bit N-PDU numbers is correspondingly performed by ignoring eight most significant bits from the value of the PDCP-PDU number. As the handover has been started, the data packets PDU are placed to a buffer to wait until the responsibility for the connection has been transferred to the serving node SGSN of another system, and the transmitted data packets can be deleted from the buffer whenever an acknowledgement of the received data packets is obtained from the recipient.
A problem in the above arrangement is the creation of N-PDU numbers from PDCP-PDU numbers. Due to a delay in the system, the buffer may contain a large number of data packets PDCP-PDU. The data packet number space that can be used for numbering data packets PDCP-PDU in the UMTS is bigger (16 bits) than the data packet number space (8 bits) used for N-PDU numbering in the GPRS system. If the number of the buffered data packets PDCP-PDU exceeds the number that can be expressed with eight bits, two or more data packets may have the same N-PDU number, as eight most significant bits are ignored in the 16 bits of the PDCP-PDU numbers. Thus, the recipient can no longer unambiguously define the original PDCP-PDU number on the basis of the N-PDU number of the received data packet, nor the data packet to be acknowledged, and the reliability of the handover cannot be ensured any more.